Very recently, SethBling and Cooper Harasyn discovered a new exploit for Super Mario World! This allows you to install a permanent jailbreak onto your console, complete with a hex editor and mod loader! Watch more about it here:

If you don't have a multitap or enough skill in Super Mario World, watch this video for instructions on installing it with a Game Genie:

Installing mods is very simple: use the hex editor on page FFFF to write in a mod, and press Select+R to activate it.Do not activate an invalid mod, and do not activate a mod unless you know it is valid. If you don't know a valid mod to enter, write 6B to the first byte in the page, which will cause the mod to do nothing.

I have created some mods myself for Super Mario World, for use with this jailbreak. If you want to try these out, or suggest ideas for mods I could code, feel free.

These are a lot of addresses, so any mod definitely won't run out of usable addresses any time soon. There are even 17 unused direct page addresses, which are shorter to represent and should be enough for all purposes. ($7E:00F8 - $7E:00FF are also unused by SMW, but they are used by the hex editor. $7E:0000 - $7E:000A are only useful as scratch space.)